We have a '98 R72 and is has always been under horse powered I thought. We have to keep RPM up by pulling back on the hydro and we struggle all the time. I've always felt we were 100 hp short. Long story on the last day of harvest this fall we found we only had 100 psi fuel pressure on the rail at normal engine load and if we pushed it as hard as we could it would go to 115-120. From what we could find out we should be 150 psi under normal engine load and raise to 175 under heavy load. In the past 3-4 years we've had pump rebuilt, all new injectors, checked for air leaks, put an electric charge pump at fuel tank to pump fuel back to the engine and replaced fuel shut off solenoid. We've cleaned and check the check valves in the fuel lines on the engine. We've ran the machine for 5 years and it's never had HP. We are going to have a cummins tech work on it this spring before wheat. We had another cummins tech look at it this fall and he was more of sham type person than really wanting to fix it. That's another story. Any one have any ideas what could be causing low fuel pressure?